Fossil fuels are burned to produce energy for a variety of applications. In many conventional engines such as gas turbines, fuel is vaporized and mixed with air at or near their stoichiometric ratio prior to combustion. In order to increase the efficiency of combustion, it is known to premix a quantity of air with the fuel prior to introduction of the mixture into the combustion zone of the engine such as the combustor of a gas turbine.
An example of premixing is disclosed in U.S. Pat. No. 4,835,962 to Rutter. Rutter discloses a fuel atomization apparatus for a gas turbine engine in which during start-up, fuel is directed through a mixing apparatus. The pressure drop created as the fuel passes through a nozzle draws air into the apparatus and effects mixing of fuel and air. However, because the flow rates through the mixing nozzle of the apparatus are not sufficient for most post-start-up engine operating conditions, it is necessary to provide a means for delivering greater amounts of fuel to the combustor.
A frequent unwanted product of combustion is oxides of nitrogen, commonly known as NOx. It is generally accepted that NOx formation results from incomplete combustion and/or very high combustion temperatures. It has been suggested that the formation of nitrous oxide (NO), a prerequisite to the formation of NO2, can be controlled either by lowering the peak temperatures in the combustor by lean, premixed operation and by exhaust gas recirculation, or by reducing the residence time in the primary zone by rapid addition of secondary air through turbulent mixing. Anand and Gouldin, Combustion Efficiency of a Premixed Continuous Flow Combustor, Journal of Engineering for Gas Turbines and Power, July 1985, Vol. 107, pp. 695-705. In order to reduce the formation of NOx, various proposals have been made to reduce the temperature of combustion, including the introduction of water into the combustion zone, low excess air operation, staged combustion or off stoichiometric firing, and flame temperature reduction.
In a liquid fuel fired engine, the water to be introduced into the combustion zone may first be emulsified with the fuel either through the addition of a chemical surfactant, the use of a mixer or other mechanical means, or both. For example, U.S. Pat. No. 4,110,973 to Haeflich et al. discloses a gas turbine engine power plant having a water/fuel mixing device that employs a fuel conduit with a helical arrangement of water injection apertures for injecting water jets through the fuel to impinge upon the inside wall of the conduit and mix homogeneously with the fuel. As an alternative to emulsification, steam may be injected directly into the combustion zone. For example, U.S. Pat. No. 4,102,125 to Schelp discloses a gas turbine engine in which water is turned to steam in the hot turbine, superheated, and then injected into the combustor to aid in the gasification of fuel and to reduce the flame temperature.
An example of staged combustion is disclosed by U.S. Pat. No. 4,629,413 to Michelson et al. Michelson teaches a premix burner having a burner tube provided with a jet eductor system at the upstream end section of the tube for inspirating and mixing primary air with fuel gas, a burner tip at the downstream end of the tube provided with ports for receiving and burning the mixture of primary air and fuel gas, and a burner tile surrounding the downstream end section of the tube. The channel between the tile and the tube section is sealed off to prevent access of secondary air thereto, and secondary air is supplied to flow downstreamwardly outside of the tile and to promote mixing of the secondary air with the flame downstream of the burner to achieve delayed combustion.
Similarly, U.S. Pat. No. 5,022,849 to Yoshii et al. discloses a low NOx burning method and apparatus wherein a low air ratio flame burned with an amount of air less than the theoretical air ratio required for burning fuel perfectly is formed, and combustibles discharged from the low air ratio flame are burned at a trailing stream of the low air ratio flame while supplying air.
However, systems for reducing the production of NOx during combustion, such as those described above, have so far been unable to meet proposed environmental regulations, or are prohibitively expensive to manufacture and operate.